The average aquarium may contain a wide range of fungal spores in the water – no matter how clean it may look – and they are certainly present in the environment outside the tank. In many cases, these spores are not a problem and do not cause disease, although they may strike if the fish sustains even a minor injury – perhaps from combat with another fish, or rough handling when it was caught.

As with all health problems, prevention is better than cure. Poor environmental conditions can foster fungal growth. Decomposing food or dead plant leaves left in the water may well cause problems, so always maintain clean, well-oxygenated water and remove debris and uneaten food.

Viral problems, by contrast, are unlikely to occur unless you add a fish to the aquarium that is already infected. This is why it is wise to isolate new fish at first, to ensure that they are healthy and do not present a hazard to those already established in the tank.

Fungal diseases

One of the advantages of a new aquarium containing treated fresh water is that it should be relatively free from fungal contamination, so that the risk of fungal diseases is at least reduced in this context. As a precautionary measure, you should always treat the site of any obvious injury to help prevent fungal infection, which would delay healing. An unattended wound is more likely to result in an infection taking hold, often indicated by a haloed effect at first, before the fungus starts to resemble cotton wool. If one or both eyes are affected, they will take on a cloudy appearance.

Treating fungal infections

There are various commercial remedies available for treating fungal infections, as well as the traditional method – a solution of malachite green which is painted directly onto any fungus developing on the body. Net the fish carefully, laying it in the net on its side on a piece of paper towel that has been dipped in the aquarium water. Keep the net slightly raised to prevent the fish from leaping out, and so that the affected area is accessible. Treat the site of the infection directly, using a small paintbrush to apply the malachite green solution with gentle dabs. This can help the healing process significantly.

Fungal attacks on eggs

In some cases, fungus may also strike the fish’s eggs, although fertilized eggs do seem to have some resistance to fungal attack. This problem is likely to be greatest in cases where a relatively high proportion of the eggs are infertile, although those of some species, such as bumblebee gobies (Brachygobias xanthozona), appear to be more susceptible to fungal infection than others. As a precaution, you can help to safeguard the eggs by treating the water with a commercial anti­fungal product. It is preferable to try to minimize problems in the first place, however: make sure that the breeding tank or hatching tank you are using for the eggs only contains fresh, treated water. This should be relatively free from fungal spores, in comparison with water that has being drawn from the main aquarium where levels of contamination and fungal spores are likely to be higher.

Viral diseases

Fortunately, relatively few viruses are of great significance as far as tropical aquarium fish are concerned. For example, carp pox is far more common in coldwater cyprinids such as goldfish (Carassius auratus), compared with their tropical relatives. There are indications that the full extent of viral infections may have been underestimated, however, and research continues on the subject. A virus has now been implicated as the likely cause of Singapore angel disease (SAD), for example, an illness with a very high mortality rate that afflicts angelfish (Pterophyllum species) bred commercially in this region.

Unfortunately, viruses use the fish’s own cell mechanisms to reproduce themselves. This makes it especially difficult to devise drugs to treat this kind of infection, simply because they are likely to disrupt normal body processes when attacking the virus -humans have the same problem in treating the common cold, which is viral in origin. Active prevention of viral disease is therefore still the favoured approach for trying to combat these illnesses at present, using vaccinations to protect susceptible fish. This also activates the fish’s immune system, helping it to overcome any subsequent viral challenge. Quarantining new fish initially to make sure they are showing no signs of illness is the best way of preventing a potential widespread viral problem in the aquarium.

Lymphocyslis

This condition is the most widespread form of viral disease encountered in tropical fish. It is possible that environmental factors, including pollution, may also affect the incidence of these tumours. An increase in the levels of heavy metals in the water has been linked with an above-average occurence of tumours in fish.

Lymphocystis causes swellings over the body surface and fins. These growths increase in size over time, often assuming a cauliflower-like appearance. Aside from disfiguring the fish, however, they do not generally cause any real harm, although this partly depends on their location on the body.

Fortunately, lymphocystis is not highly infectious, but if an infected individual comes into close contact with another fish that has an injury, there is a much greater likelihood that the disease will be transmitted. In any case, you should isolate any affected individuals to prevent the possible spread of infection. There is no effective treatment, although there have been claims that abrading the lesion may be successful; it is thought this may help to trigger the fish’s immune response, enabling it to overcome the infection.

In a few cases, lymphocystis may also strike the internal body organs; the growth usually first becomes apparent by exerting pressure from within, interrupting the natural profile of the fish’s body. In this instance, where the tumour may well affect how the internal organs function, euthanasia may be necessary.

Malawi bloat

This remains rather a mysterious illness, which is associated with cichlids originating from Lake Malawi and neighbouring East African lakes, as its name suggests. A virus has been suspected as the cause, but it is possible that other factors could be responsible as well. The symptoms resemble those of dropsy, with the fish developing a swollen body. The scales become lifted and raised away from the body, rather than forming a sleek body casing. Fish showing these signs should be set apart in an isolation tank. In some cases, there may also be other symptoms – pop-eye or exophthalmia is sometimes seen with this condition, and fluid may accumulate in the body cavity. In such cases, the prognosis is poor.

About the Author: Fred Goodson has a passion for pets and animals. He has 4 dogs and is planning to have another one. He is also a blogger who writes about pets and animals. Currently, he is living in New Jersey.